The invention relates generally to a method and a plant for processing milk and, in particular, to a method and a plant for producing a sterile milk product.
Milk processing typically involves several steps, including separation of raw milk into a milk portion and a cream portion, filtering the milk portion to form a permeate and a retentate, heating and/or filtering the permeate to reduce the number of live microorganisms, heating the cream to reduce the number of live microorganisms, recombining the treated cream and treated milk, homogenizing the combined milk and cream, and packaging the resultant milk product.
In order to increase the efficiency of milk processing and reduce costs, it is desirable that the amount of raw milk lost or discarded during each step of milk processing be minimized.
Raw milk naturally contains various microorganisms such as bacteria, yeast and mold. Additionally, raw milk may come into contact with and retain additional microorganisms during storage and transport. These microorganisms can contribute to the degradation and eventual spoilage of milk. It is therefore desirable to remove or deactivate these microorganisms.
Microorganisms such as bacteria can be categorized by size and/or heat sensitivity. Heat resistant bacteria are bacteria which can be deactivated by, for example, heating to a temperature of from at least about 140xc2x0 C. to about 150xc2x0 C. for about 4 to about 6 seconds as known to practitioners in the art. This category of bacteria is normally referred to as spores, and includes bacteria of species Bacillus and Clostridium. These bacteria, as defined by their least diameter, are about 0.5 xcexcm or larger. The smallest heat resistant bacterium identified in milk by the inventor is Bacillus pumilis, which has a least diameter of about 0.5 xcexcm.
Thermoduric bacteria are bacteria which are deactivated by, for example, heating to temperatures of about 100xc2x0 C. for a few seconds, or equivalent time and temperature combinations as known to practitioners in the art. The smallest thermoduric bacterium identified in milk by the inventor is microbacterium Lacticum, which has a least diameter of about 0.3-0.4 xcexcm. Thus, these bacteria have a least diameter of about 0.3 xcexcm or larger.
Heat sensitive bacteria are bacteria which are deactivated by, for example, heating to temperatures as low as about 72xc2x0 C. to about 75xc2x0 C. for about 15 to about 20 seconds, or equivalent time and temperature combinations as known to practitioners in the art. The smallest heat sensitive bacterium has a least diameter of less than about 0.3 xcexcm.
Raw milk also contains naturally occurring enzymes, as well as extracellular enzymes produced by microorganisms, such as psychrotrophic bacteria, which are typically present in milk. Certain of these enzymes are known to be bound to somatic cells which typically have a least diameter larger than about 0.5 xcexcm. These enzymes can also contribute to the degradation and eventual spoilage of milk.
It is desirable to remove or deactivate the above types of bacteria and enzymes, as well as other microorganisms, in order to prevent degradation of the raw milk and/or resultant milk products. Typically, microorganisms and enzymes are deactivated by heat treatments and/or removed by filtration of the raw milk. The use of heat treatments alone, however, can impart an undesirable cooked flavor to the milk due to the high temperatures (e.g., 140-150xc2x0 C.) required to deactivate heat resistant microorganisms. Also, filtration alone does not remove all live microorganisms present in the raw milk.
Combinations of filtration and heat treatment have been used to provide a purer milk product wherein more of the microorganisms are removed or deactivated than would occur using either heat treatment or filtration alone. See, for example, WO 98/57549 and U.S. Pat. No. 5,935,632.
Filtration produces a retentate in which the microorganisms are highly concentrated. The concentration of microorganisms in the retentate can be, for example, at least about 10 to 100 times higher than in the raw milk. This retentate typically comprises from at least about 1% to about 10% of the total incoming raw milk. Multiple filtrations of the retentate produce a final retentate in which the microorganisms are even more concentrated, but wherein less milk is retained in the retentate, typically about 1% or less. See, for example, WO 98/57549.
The retentate from filtration can be recycled into the milk or cream processing stream as described, for example, in U.S. Pat. No. 5,935,632, wherein the retentate is added to the cream portion for processing with the cream, or in U.S. Pat. No. 5,683,733, wherein the retentate is added to the milk processing stream before the milk processing stream is fed to the separator. In either case, the number of microorganisms in the cream portion or in the resultant milk product is increased. The retentate can also be discarded as waste, as described, for example, in WO 98/57549. However, this results in a lower yield.
There thus remains a need in the art to develop a process by which the concentration of microorganisms in the milk product can be reduced while the yield or the percentage of raw milk which becomes final milk product is increased.
It is an object of this invention to provide a process and apparatus for producing a sterile milk product that overcomes the deficiencies of the prior processes and apparatus. The process in accordance with a preferred embodiment of the invention includes: filtering a skim milk portion through a primary filter assembly and through a secondary filter assembly. The primary filter assembly includes individual filters arranged in series. The individual filters each have a microfilter that retains at least a portion of the microorganisms and passes the remaining fluid through the filter. Thus, each filter in the series produces a permeate stream and a retentate stream. The next filter in the series receives the retentate from the preceding filter and produces a permeate stream that contains fewer microorganisms and a retentate stream that contains progressively more microorganisms. Each filter in the primary filter assembly provides a permeate that has substantially fewer microorganisms than are presented in the skim milk portion that is supplied to the first filter of the primary assembly.
The permeate stream from the individual filters of the primary filter assembly is supplied to a heat treatment operation or other appropriate treatment to produce a stable milk product having a long shelf life.
The total retentate discharge from the first assembly flows to the secondary filter assembly which contains one or more individual filters. In the secondary filter assembly, the permeate stream from each individual filter is collected and conducted selectively to the supply conduit for the skim milk portion, or to the standardization unit, or other suitable processing step. The retentate stream from the first filter is fed into the next filter and the retentate from the last filter in the second assembly is conducted to an appropriate disposal site.
The filters in the second filter assembly preferably have equal or bigger pore size than the last filter in the primary filter assembly. The individual filters in the primary filter assembly preferably have equal or progressively smaller pore sizes from one individual filter to the next in the series. Preferably, the first individual filter has a pore diameter that is sufficiently large to retain all microorganisms having a least diameter of less than or equal to 0.5 xcexcm. Bacteria having a relatively larger least diameter are retained by the filter as the skim milk portion passes through the filter into the permeate stream. The retentate is conducted to the next filter. This process is repeated at each filter in the series. In this manner, the concentration of bacteria in the retentate becomes progressively larger for each filter in the series. In the secondary filter assembly, the individual filters have equal or progressively smaller pore sizes from one filter to the next in a series, so that additional bacteria is removed and the permeate can be supplied to the raw milk stream, or the skim milk portion, or the cream portion.